Cathodic corrosion and dry fire protection apparatus and methods for electric water heaters

ABSTRACT

The metal tank portion of an electric water heater is protected against corrosion utilizing a corrosion protection system that detects a voltage potential between the sheath portion of a tank water-immersed electric heating element and the tank. In one embodiment of the corrosion protection system the sensed sheath/tank potential is utilized to enable a user of the water heater to accurately gauge the necessity of replacing a sacrificial anode extending into the tank. In another corrosion protection system, the sensed sheath/tank potential is utilized to provide impressed current cathodic protection of the tank and also to prevent dry firing of the electric water heater.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of and claimspriority to U.S. patent application Ser. No. 14/305,581 filed Jun. 16,2014, which claims the benefit of the filing date of provisional U.S.patent application No. 61/838,749 filed Jun. 24, 2013. The entiredisclosures of the foregoing applications are hereby incorporated hereinby this reference.

BACKGROUND OF THE INVENTION

In a typical electric water heater, or in other types of electric liquidheating apparatus, the liquid to be heated is disposed in a vessel intowhich a resistance-type submersible electric heating element projectsand may be selectively energized to heat the liquid to a predeterminedtemperature. As commonly manufactured, the vessel is of a ferrous metalmaterial, such as steel, which is lined with a protective ceramic orother material to inhibit corrosion of the metal which can lead toleakage of the vessel. Corrosion is essentially an electrochemicalphenomenon, and cathodic protection is a commonly used method ofcombating it.

The most common techniques for providing such cathodic protection forthe liquid-containing vessels or tanks of electric liquid heatingapparatus are (1) utilizing an erodable sacrificial anode in the vessel,and (2) using an impressed current cathodic protection (ICCP) anode inthe vessel. The sacrificial anode technique has the advantages of lowcost, continuous protection without any limitations such as poweroutage, software malfunction, etc. However, it has a limited life, andmust be inspected periodically to verify that it has not dissolved to anextent rendering it incapable of carrying out its cathodic protectionfunction. The ICCP anode protection technique provides severaladvantages over the sacrificial anode approach to cathodic corrosionprotection, such as being capable of being precisely controllable withrespect to its potential and current outputs, and having potentiallyunlimited operational life. However, the ICCP anode protection techniquehas some disadvantages such as requiring an associated control systemand being more complex and costly than a sacrificial anode system.

As can be seen from the foregoing, it would be desirable to provide anelectric liquid heating apparatus having incorporated therein improvedcathodic corrosion protection apparatus, based on either a sacrificialanode approach or an impressed corrosion inhibiting voltage approach. Itis to this goal that the present invention is primarily directed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electric water heater incorporatingtherein tank corrosion protection apparatus embodying principles of thepresent invention and including a sacrificial anode and a speciallydesigned electric resistance type immersion heating element andassociated control apparatus, having the illustrated control steps, formonitoring the depletion of the sacrificial anode;

FIG. 2 is a longitudinally foreshortened side elevational view of thespecially designed heating element of FIG. 1;

FIG. 3 is an enlarged scale cross-sectional view of the portion of theFIG. 2 heating element within the dashed area “3”, with the heatingelement operatively installed in an electric water heater partiallyillustrated in phantom in FIG. 3;

FIG. 4 is a schematic diagram of an alternate electric water heaterembodiment incorporating therein combination tank corrosion/dry fireprotection apparatus embodying principles of the present invention andincluding the electric heating element of FIGS. 2 and 3 and associatedcontrol apparatus; and

FIG. 5 is a schematic logic flow diagram illustrating the operation ofthe control apparatus of FIG. 4

DETAILED DESCRIPTION

Referring initially to FIG. 1, in a first representative embodimentthereof the present invention provides an electric liquid heatingapparatus which is illustratively an electric water heater 10, but couldalternately be another type of electric liquid heating apparatus, suchas an electric boiler, without departing from principles of the presentinvention. The electric water heater 10 has a storage vessel in the formof a ferrous metal tank 12, illustratively of a steel construction,having the usual protective ceramic lining (not shown). As is well knownin the water heating art, despite the presence of this protectivelining, the metal tank 12 is subject to corrosion as its lining wearsaway over the operational life and/or the pre-existing defect of thelining of the tank 12. Extending inwardly through the tank wall 12 forheating water 14 in the tank 12 are upper and lower submersibleresistance type electric heating elements 16,18 that may be selectivelyenergized to heat the water 14.

Turning now to FIGS. 2 and 3, the upper electric heating element 16 mayhave a specially designed configuration embodying principles of thepresent invention. The electric heating element 16 has an elongated,generally U-shaped hollow metal sheath structure 20 with spaced apartparallel legs 22 and 24 having unjoined outer end portions 22 a, 24 aand inner end portions 22 b, 24 b joined by a curved sheath section 20c. Representatively, but not by way of limitation, the metal material ofthe sheath structure 20 is titanium. Extending through the interior ofthe sheath structure 20 is an electric heating filament 26 which iselectrically isolated from the sheath structure 20 by suitableinsulation material 28 within the sheath structure. The outer sheathends 22 a, 24 a extend through corresponding openings in an externallythreaded cylindrical metal mounting plug 30 which, with the sheath 20inserted into the tank interior through an internally threaded mountinghole 32 in the metal tank wall 12, is threaded into the mounting hole32. As depicted in FIG. 3, the outer sheath ends 22 a, 24 a arecircumscribed by insulating structure illustratively in the form oftubular insulating members 34 (representatively of a plastic material)which extend through the metal mounting plug 30, thereby electricallyisolating the sheath 20 from both the mounting plug 30 and the tank 12.

An insulative housing or cover 36, representatively of a plasticmaterial, is suitably secured to the outer side of the metal mountingplug 30 and receives outwardly projecting portions of the outer sheathleg ends 22 a, 24 a (see FIG. 3). Electrically conductive powerconnection pins 38 extend inwardly through the housing 36 and into theouter ends 22 a, 24 a of the sheath 20 wherein the power connection pins38 are operatively coupled to the opposite ends of the electric heatingfilament 26.

The power connection pins 38 form a first connection structure, coupledto the electric heating filament 26, through which electrical heatingcurrent may be selectively flowed to generate heat used to heat thewater 14 in the tank 12. A second connection structure is provided inthe form of a metal ring 40 circumscribing the outer sheath leg end 24 aand in electrically conductive contact therewith (and electricallyisolated from the power connection pins 38), and a metal connector screw42 is threaded through the housing 36 into electrically conductivecontact with the metal ring 40 and thus in electrical communication withthe sheath 20. Through the metal connector screw (via an electrical wireconnected thereto) an electrical voltage may be impressed on the sheathstructure 20, or a voltage potential between the sheath structure 20 andthe metal tank 12 may be sensed, for purposes subsequently describedherein.

Returning to FIG. 1, in accordance with principles of the presentinvention, the electric water heater 10 is provided with a speciallydesigned protection apparatus 44 operative to inhibit corrosion of thetank 12. The protection apparatus 44 includes a dissolvable protectiveanode 46, representatively a magnesium anode, the electricallyconductive sheath structure 20 of the upper electric heating element 16,a sensing portion 48, and a control portion 50. Basically, the upperelectric heating element sheath 20, the sensing portion 48 and thecontrol portion 50 facilitate the corrosion protection of the tank 12 bysensing and indicating the degree of depletion of the sacrificial anode46 to enable a user of the water heater 10 to accurately gauge thenecessity of replacing the sacrificial anode 46 without removing andinspecting it, and to avoid excessive tank corrosion caused byundetected excessive sacrificial anode depletion.

The sensing portion 48 includes a volt meter 52 connected in parallelwith a resistor 54 across electrical leads 56,58 respectively connectedto the tank 12 and the sheath 20 of the upper electric heating element16. For purposes of illustrative clarity, the lead 58 has beenschematically depicted in FIG. 1 as being connected to the upper heatingelement sheath 20 inside the tank 12. However, it will be readilyappreciated by those of skill in this particular art that in practicethe lead 58 would be connected to the upper heating element metalconnector screw 42 outside the tank 12 (see FIG. 3).

During operation of the water heater 10 (with water 14 in the tank 12)electrical current C1 flows from the sacrificial anode 46 to the tank12, and electrical current C2 flows sequentially from the sacrificialanode 46 to the upper heating element sheath 20, the electrical lead 58,across the resistor 54, and then into the tank 12 via the electricallead 56. The purpose of the resistor 54 is to limit the magnitude of theelectrical current C2 in a manner such that the electrical current flowfrom the sacrificial anode 46 is predominately the current C1 from thesacrificial anode 46 directly to the tank 12.

With continuing reference to FIG. 1, the volt meter portion 52 of thesensing portion 48 of the protection apparatus 44 detects the voltageacross the resistor 54 and outputs a voltage signal 60 indicative of thedetected voltage. As the sacrificial anode 46 is progressively depleted,the electrical current C2 progressively diminishes, thereby creating acorresponding reduction in the voltage across the resistor 54. Thevoltage drop across the resistor is thus indicative of the degree oferosion/depletion of the sacrificial anode 46.

The control portion 50 of the protection apparatus 44, by means ofsuitable electrical circuitry such as a pre-programmed microprocessor orthe like (not shown), utilizes the voltage signal 60 to enable a user ofthe water heater 10 to accurately gauge the necessity of replacing thesacrificial anode 46 without removing and inspecting it, and to avoidexcessive tank corrosion caused by undetected excessive sacrificialanode depletion. The manner of such utilization of the voltage signal 60entails the following steps depicted in the schematic logic flow diagramportion of FIG. 1 which will now be described.

At step 62 the voltage signal 60 is received by the control portion 50and measured, and a transfer is made to step 64 at which a query is madeas to whether the measured voltage is greater than a predeterminedmagnitude (representatively 0.5 volts). If it is, a transfer is made tostep 66 at which a suitable display is created indicating to the waterheater user or technician that the sacrificial anode 46 is functional.If at step 64 it determined that the measured voltage is not greaterthan the first predetermined magnitude thereof, a transfer is made tostep 68 at which the voltage across the resistor 54 is re-measured and atransfer is made to step 70 at which a query is made as to whether there-measured voltage is less than a second predetermined magnitude(representatively 0.2 volts). If it is not, a transfer is made to step72 at which a suitable display is created indicating to the water heateruser or technician that the sacrificial anode 46 has been depleted tosuch an extent that it should be replaced soon. If the query answer atstep 70 is “yes”, a transfer is made to step 74 at which a suitabledisplay is created indicating to the water heater user or technicianthat the sacrificial anode 46 has been depleted to such an extent thatit should be immediately replaced.

Schematically illustrated in FIG. 4 is an alternate embodiment 10 a ofthe previously described electric water heater 10. For ease in comparingthe water heater embodiments 10 and 10 a, elements in the water heater10 a similar to those in water heater 10 have been given the samereference numerals to which the subscript “a” has been added.

Water heater 10 a has a corrodible lined ferrous metal tank 12 a intothe interior of which upper and lower electric heating elements 16 a, 18a project, the heating elements 16 a, 18 a having the electricallyconductive metal sheaths 20 a and associated tubular insulating members34 a. Unlike the previously described water heater 10, the water heater10 a is not provided with a sacrificial anode. Instead, a protectionapparatus 70 associated with the water heater 10 a utilizes the upperheating element sheath 20 a to provide the tank 12 a of the water heater10 a with impressed current cathodic protection to inhibit corrosion ofthe tank 12 a. The protection apparatus 70 also functions to prevent dryfiring of the upper electric heating element 16 a.

In addition to the upper heating element electrically conductive sheath20 a, the protection apparatus 70 includes a control portion in the formof a control circuit 72 that receives DC electrical power via electricallead 74 from an AC/DC convertor 76 fed with 240 volt AC power via linesL1 and L2. An electrical lead 78 has a voltage current limiter 80therein and is connected to the upper heating element sheath 20 a (viathe connection screw 42 of the upper heating element 16 a as shown inFIG. 2). An electrical lead 82 is connected to the tank 12 a and is alsocoupled to a sensing portion of the protection apparatus 70 in the formof a potential sensing circuit 84. An additional electrical lead 83connects the electrical lead 78 to the potential sensing circuit 84 tosense potential difference between the tank 12 a and the heating elementsheath 20 a.

Direct electrical current output from the control circuit 72 viaelectrical lead 78 sequentially flows through the current limiter 80into the upper heating element sheath 20 a, to the tank via current flowC through the tank water 14 a, and then into the potential sensingcircuit 84 which responsively transmits to the control circuit 72 asignal 86 indicative of the magnitude of the sensed voltage potentialbetween the upper heating element sheath 20 a and the tank 12 a.

FIG. 5 schematically depicts a logic flow chart illustrating the mannerin which the control circuit 72 regulates the operation of theprotection apparatus 70 to inhibit corrosion of the water heater tank 12a. Upon starting the operation of the protection apparatus 70 at step88, with the operational voltage of the protection apparatus 70 turnedoff, at step 90 the control circuit 72 checks the voltage potential atthe upper heating element sheath 20 a as represented by the magnitude ofthe voltage signal 86 received by the control circuit 72 and then makesa query at step 92 as to whether a voltage potential above apredetermined threshold magnitude has been detected. If it has not, atransfer is made to step 94 at which an error signal indicative of aheating element dry fire condition is generated, such error signal beinguseable to terminate or prevent energization of the upper and lowerheating elements 16 a and 18 a.

If at step 92 a voltage potential above the predetermined thresholdmagnitude (which is representatively the “natural” voltage potentialbetween the submerged upper heating element sheath 20 a and the tank 12a without the impression thereon of voltage from the external voltagesource), a transfer is made to step 96 at which the detected potentialis recorded. Next, at step 98 the protection apparatus supply voltage isturned on and the control circuit 72 outputs a DC voltage equal to therecorded voltage potential plus a predetermined protective voltagemagnitude (representatively 1.5 volts DC). Then, at step 100 apredetermined delay period is initiated. After the predetermined delayperiod expires, a transfer is made to step 102 at which the protectionapparatus supply voltage is turned off again and a transfer is made backto step 90 to permit the system to again cycle through the operationalsteps 90 through 102.

As can be seen from the foregoing, the present invention providessubstantial improvements in the cathodic protection of corrodible vesselportions of electric liquid heating apparatus, in both sacrificial anodeand impressed current corrosion protection versions thereof. Aspreviously mentioned, although the foregoing description is directed toelectric water heater apparatus, the representatively illustratedcathodic protection apparatus and methods of the present invention arenot limited to electric water heaters, but may alternatively be used invarious other types of electric liquid heating apparatus withoutdeparting from principles of the present invention.

Additionally, various modifications can be made to the representativelyillustrated liquid heating apparatus without departing from principlesof the present invention. For example, and not by way of limitation,various other techniques could be utilized to sense the voltagedifferential between a heating element sheath and an associatedcorrodible liquid containment vessel. Moreover, while each of therepresentative electric water heaters 10 and 10 a has been shown ashaving two electric heating elements, a greater or smaller number ofelectric heating elements could be used without departing fromprinciples of the present invention. Furthermore, where a plurality ofelectric heating elements are utilized in a given electric heatingapparatus vessel, not all of the electric heating elements would have tobe configured as shown in FIGS. 2 and 3. Also, other techniques could beutilized to electrically isolate one or more of the depicted electricheating element sheaths from the metal tank.

The foregoing detailed description is to be clearly understood as beinggiven by way of illustration and example only, the spirit and scope ofthe present invention being limited solely by the appended claims.

What is claimed is:
 1. An electric heating element for insertion into aliquid storage vessel of a liquid heating apparatus through a mountingopening in a wall portion of the liquid storage vessel, the electricheating element comprising: a mounting portion connectable in themounting opening to the liquid storage vessel; an electricallyconductive hollow sheath structure having an outer end secured to themounting portion, and an inner end spaced apart from the outer end andbeing insertable through the mounting opening into the interior of theliquid storage vessel; an electric heating filament extending throughthe interior of the electrically conductive hollow sheath structure andbeing electrically isolated therefrom; an insulating structureassociated with the outer end of the electrically conductive hollowsheath structure and operative to electrically isolate the outer end ofthe electrically conductive hollow sheath structure from the liquidstorage vessel when the electrically conductive hollow sheath structureis installed in the liquid storage vessel; a first connection structure,coupled to the electric heating filament, through which electricalheating current can flow to the electric heating filament; and a secondconnection structure, electrically isolated from the first connectionstructure, through which an electrical voltage may be impressed on theelectrically conductive hollow sheath structure.
 2. The electric heatingelement of claim 1 wherein: the electrically conductive hollow sheathstructure has an elongated, generally U-shaped configuration.
 3. Theelectric heating element of claim 1 wherein: the second connectionstructure includes an annular, electrically conductive connection memberin electrically conductive communication with the outer end of theelectrically conductive hollow sheath structure, and a connection screwthreaded into the annular, electrically conductive connection member. 4.The electric heating element of claim 1 wherein: the insulatingstructure extends through the mounting portion.
 5. The electric heatingelement of claim 1 wherein the electric heating element is configured tobe coupled to a protection apparatus.
 6. The electric heating element ofclaim 5, wherein the protection apparatus comprises: a sensing portionconfigured to measure a voltage between the liquid storage vessel andthe electrically conductive hollow sheath structure when the electricheating element is operating and installed in the liquid storage vessel;and a control portion configured to determine whether a sacrificialanode installed within the liquid storage vessel is depleted.
 7. Theelectric heating element of claim 6, wherein the control portion isfurther configured to sequentially (1) measure the voltage between theliquid storage vessel and the electrically conductive hollow sheathstructure in the absence of the electrical voltage that may be impressedon the electrically conductive hollow sheath structure, (2) record amagnitude of the voltage measured if the voltage measured is above apredetermined value, (3) apply the electrical voltage that may beimpressed to the electrically conductive hollow sheath structure equalin magnitude to the magnitude of the voltage recorded plus apredetermined additional impressed voltage, and (4) terminate theelectrical voltage that is applied to the electrically conductive hollowsheath structure, and restart the sequence.
 8. The electric heatingelement of claim 5, wherein the protection apparatus comprises: asensing portion configured to sense a voltage potential between theliquid storage vessel and the electrically conductive hollow sheathstructure when the electric heating element is operating and installedin the liquid storage vessel; and a control portion coupled to thesensing portion of the protection apparatus and operative to measure thevoltage potential sensed between the electrically conductive hollowsheath structure and the liquid storage vessel and: (1) if the voltagepotential measured is greater than a first predetermined magnitude,provide an indication that a sacrificial anode is still functional, and(2) if the voltage potential measured is not greater than the firstpredetermined magnitude, re-measure the voltage potential sensed betweenthe electrically conductive hollow sheath structure and the corrodiblevessel, compare the voltage potential that is re-measured to apredetermined second magnitude, and then provide at least one indicationof a degree of necessity of replacing the sacrificial anode based on thecomparison of the voltage potential that is re-measured and thepredetermined second magnitude.
 9. The electric heating element of claim6, wherein the sensing portion of the protection apparatus includes aresistor and a voltage measuring device operative to sense the voltagepotential between the electrically conductive hollow sheath structureand the liquid storage vessel.
 10. The electric heating element of claim5, wherein the protection apparatus includes apparatus for selectivelyimpressing a voltage on the electrically conductive hollow sheathstructure from an external voltage source, and the protection apparatusfunctions to provide impressed current cathodic protection to the liquidstorage vessel without utilization in the liquid storage vessel of aseparate anode structure for such function.
 11. The electric heatingelement of claim 5, wherein the protection apparatus further includes acontrol portion coupled to the sensing portion of the protectionapparatus and operative to sequentially (1) measure a voltage potentialbetween the electrically conductive hollow sheath structure and theliquid storage vessel, in the absence of an impressed voltage on theelectrically conductive hollow sheath structure, and then (2) output asignal indicative of a dry firing condition of the electric heatingelement if the voltage potential measured between the electricallyconductive hollow sheath structure and the liquid storage vessel is notabove a predetermined value thereof.
 12. An electric heating element forinsertion into a liquid storage vessel of a liquid heating apparatusthrough a mounting opening in a wall portion of the liquid storagevessel, the electric heating element comprising: a mounting portionconnectable in the mounting opening to the liquid storage vessel; anelectrically conductive hollow sheath structure having an outer endsecured to the mounting portion, and an inner end spaced apart from theouter end and being insertable through the mounting opening into theinterior of the liquid storage vessel; an electric heating filamentextending through the interior of the electrically conductive hollowsheath structure and being electrically isolated therefrom; aninsulating structure associated with the outer end of the electricallyconductive hollow sheath structure and operative to electrically isolatethe outer end of the electrically conductive hollow sheath structurefrom the liquid storage vessel when the electrically conductive hollowsheath structure is installed in the liquid storage vessel; a firstconnection structure, coupled to the electric heating filament, throughwhich electrical heating current can flow to the electric heatingfilament; and a second connection structure, electrically isolated fromthe first connection structure, through which an electrical current maybe impressed on the electrically conductive hollow sheath structure. 13.The electric heating element of claim 12 wherein: the electricallyconductive hollow sheath structure has an elongated, generally U-shapedconfiguration.
 14. The electric heating element of claim 12 wherein: thesecond connection structure includes an annular, electrically conductiveconnection member in electrically conductive communication with theouter end of the electrically conductive hollow sheath structure, and aconnection screw threaded into the annular, electrically conductiveconnection member.
 15. The electric heating element of claim 12 wherein:the insulating structure extends through the mounting portion.
 16. Theelectric heating element of claim 12 wherein the electric heatingelement is configured to be coupled to a protection apparatus.
 17. Theelectric heating element of claim 16, wherein the protection apparatuscomprises: a sensing portion configured to measure a voltage between theliquid storage vessel and the electrically conductive hollow sheathstructure when the electric heating element is operating and installedin the liquid storage vessel; and a control portion configured todeliver the electric current to the electrically conductive hollowsheath structure.
 18. The electric heating element of claim 17, whereinthe control portion is further configured to sequentially (1) measurethe voltage between the liquid storage vessel and the electricallyconductive hollow sheath structure, (2) record a magnitude of thevoltage measured if the voltage measured is above a predetermined value,(3) apply a protective electrical voltage that may be impressed to theelectrically conductive hollow sheath structure equal in magnitude tothe magnitude of the voltage recorded plus a predetermined additionalimpressed voltage, and (4) terminate the electrical voltage that isapplied to the electrically conductive hollow sheath structure, andrestart the sequence.
 19. The electric heating element of claim 16,wherein the protection apparatus includes apparatus for selectivelyimpressing a voltage on the electrically conductive hollow sheathstructure from an external voltage source, and the protection apparatusfunctions to provide impressed current cathodic protection to the liquidstorage vessel without utilization in the liquid storage vessel of aseparate anode structure for such function.
 20. The electric heatingelement of claim 16, wherein the protection apparatus further includes acontrol portion coupled to the sensing portion of the protectionapparatus and operative to sequentially (1) measure a voltage potentialbetween the electrically conductive hollow sheath structure and theliquid storage vessel, in the absence of an impressed voltage on theelectrically conductive hollow sheath structure, and then (2) output asignal indicative of a dry firing condition of the electric heatingelement if the voltage potential measured between the electricallyconductive hollow sheath structure and the liquid storage vessel is notabove a predetermined value thereof.